1. Technical Field
The present invention relates to a CLEP (Code Excited Linear Prediction) voice coder (or, called as vocoder) for improving process time and speech quality of G.723.1 and reducing bit rate.
2. Description of the Prior Art
Generally, CELP (Code Excited Linear Prediction) is a method most broadly used in the vocoder field. This method may obtain good speech quality at about 4.8 kbps bit rate and has been standardized with several standardizing organizations in various applications.
Such method is applicable to an internet phone, a video conference, a voice mail system, a voice pager, etc. and currently TRUE SPEECH and G.723.1 voice coder (called also as xe2x80x9cvocoderxe2x80x9d) are commonly used as a commercial version.
Among them, G.723.1 shown in FIG. 1 has a dual bit rate of 5.3/6.3 kbps, which is used in the internet phone, commercially used as special communication means now, and in a communications vocoder. G.723.1 provides good quality in comparison with its low bit rate. In addition, G.723.1 is more applicable than other vocoder standards because it uses two bit rates for optimized transmission circumstance.
However, because G.723.1 uses an analysis method using composition of the CELP vocoder, which is a manner of separating and then composing components of a voice signal, there is an unavoidable problem of time consumption due to its high computational complex.
In addition, because G.723.1 Dual Bit Rate Speech Codec includes different vocoders, many internal memories and much computational complex are required when realizing it with DSP (Digital Signal Processor) chips. Particularly, because MP-MLQ (Multi Pulse Maximum Likelihood Quantization) mode requires more computational complex than ACELP (Algebraic CELP), the vocoder algorithm which requires less algorithm computational complex to use an inexpensive DSP, is more suitable in the internet phone.
In addition, because, among VAD (Voice Activity Detector) and CNG (Comfortable Noise Generator) used to reduce a bit rate in a voice inactive interval, the VAD uses only energy parameter for final determination of voice activity, there is a drawback that accurate VAD determination is difficult during the energy critical value reaches a current energy level or when SNR is a low signal. Moreover, in fact that G.723.1 vocoder employs a pitch/formant post-filter for improvement of speech quality in a decoding terminal, in which the post-filter uses only the first degree slope compensation filter and the pitch post-filter performs search process under the condition that energy levels are equal in every pitch interval, there is a problem that accurate pitch search is hardly obtained in an interval where the energy level changes.
The present invention is designed to solve the problem of the prior art. An object of the present invention is to provide a search method, which reduces a processing time of a vocoder by determining GRID BIT of ML-MLQ (Multi Pulse Maximum Likelihood Quantization) in advance.
Another object of the present invention is to provide a search method, which improves speech quality by using a formant post-filter and a pitch post-filter for searching a pitch through energy level standardization as multi-degree slope compensation filters.]
Still another object of the present invention is to provide a search method, which reduces a bit rate in a voice inactive interval by using an algorithm for simply determining a SID (Silence Insertion Descriptor) frame with a ZCR (Zero Crossing Rate) parameter when determining VAD and SID frames having a LSP (Line Spectrum Pair), a pitch gain and energy parameter.
In order to obtain the above object, the present invention suggests a method of searching MP-MLQ fixed codebook through bit predetermination including the steps of generating a target vector with amplitude, reducing time to search an optimal pulse array through the bit predetermination and searching all of pulses if two errors have an identical value; a formant post-filtering method of extracting a reflection coefficient of a slope compensation filter to apply a multi-degree slope compensation thereto; a pitch post-filtering method including an energy level standardization step and a step of generating a signal approximate to an average energy level; a VAD algorithm method using an energy, a pitch gain and a LSP distance; and a method of enhancing a processing time of G.723.1, improving speech quality and reducing a bit rate by using a determination logic algorithm in setting a SID frame for the voice inactive interval, and a CELP vocoder using one of the methods.